Thermoplastic polyesters typified by poly(ethylene terephthalate) have excellent physical and chemical properties and accordingly are in wide use in film fields such as magnetic tape application, electrical insulation application, condenser application, photography application, packaging application and the like.
Polyester films are usually produced by extruding a film-like melt from an extruder die, rapidly cooling the melt on the surface of a rotating cooling drum, then biaxially stretching the resulting film to lengthwise and crosswise directions. In this case, in order to obtain a film having no surface defect and a thickness of high uniformity, it is necessary to increase the adhesion between the film-like melt and the surface of the rotating cooling drum. As the method for increasing the adhesion, there is known a method in which a wire-shaped electrode is provided between the extruder die and the surface of the rotating cooling drum to deposit electrostatic charges on the surface of the film-like melt and thereby the film-like melt is allowed to adhere to the surface of the cooling drum and is cooled rapidly under this condition (the method is hereinafter referred to as electrostatic casting method) (reference is made to Japanese Patent Publication No. 6142/1962).
Even in this electrostatic casting method, however, as the circumferential speed of the rotating cooling drum is made faster to increase the productivity of film, the strength of the adhesion of the film-like melt onto the surface of the cooling drum is reduced, inviting the formation of pinner bubbles on the film surface and reducing the uniformity of film thickness.
In film formation, particularly the formation of a monoaxially or biaxially stretched film, higher productivity and resultant reduced production cost is an important task, together with higher film quality. To achieve it, employing a faster circumferential speed of the rotating cooling drum and thereby increasing a speed of film formation is most effective. In the electrostatic casting method, when the circumferential speed of the rotating cooling drum is made faster, the amount of electrostatic charges applied per unit area of the surface of film-like melt becomes smaller, and thereby the adhesion between film-like melt and drum surface is reduced, which invites formation of pinner bubbles on film surface and reduction in uniformity of film thickness. Hence, in order to increase the adhesion, there can be taken a means for increasing the amount of electrostatic charges deposited on the film-like melt surface by increasing the voltage applied to the above-mentioned electrode. Application of too high a voltage to the electrode gives rise, in some cases, to arcing between the electrode and the rotating cooling drum, breakage of the film-like melt on the cooling drum surface and damaging of the cooling drum surface. Thus, increase of the voltage applied to the electrode to higher than a certain level is virtually impossible and the conventional electrostatic casting method has a limitation for obtaining a uniform film by increasing the speed of film formation.
To produce a polyester film having a uniform thickness and no surface defect at a high efficiency by overcoming such a limitation of the electrostatic casting method and using a higher film formation speed, Japanese Patent Publication No. 40231/1978 proposes incorporation into a polymer an alkali metal, an alkaline earth metal or a compound thereof in an amount of 0.005-1% by weight, preferably 0.07-0.3% by weight, to allow the resulting molten polymer to have a resistivity of 2.times.10.sup.7 -5.times.10.sup.8 .OMEGA..multidot.cm to increase the above-mentioned adhesion. The document shows, as specific examples, addition of 0.15% by weight of lithium acetate to allow the molten polymer to have a resistivity of 0.5.times.10.sup.8 .OMEGA..multidot.cm and addition of 0.09% by weight of strontium acetate to allow the molten polymer to have a resistivity of 2.times.10.sup.8 .OMEGA..multidot.cm.
In the direct polymerization process for obtaining a polyester, when an alkali metal compound or an alkaline earth metal compound is added, the amount of diethylene glycol (DEG) formed as a by-product is large and the obtained polyester has a lower melting point, which invites various problems, for example, easy sticking of film onto casting drum and increased breakage of film during stretching. Therefore, as a process for producing a polyester wherein the amount of DEG formed as a by-product is small and the electrostatic castability is improved, Japanese Patent Publication No. 15730/1981 proposes adding, after the substantial completion of esterification reaction, a metal compound selected from Zn, Mg and Mn compounds and a phosphorus compound so as to satisfy the following formulas EQU 20.ltoreq.M.ltoreq.1,000 EQU 0.8.ltoreq.M/P.ltoreq.5
[M indicates the total metal atom amount (ppm) of the metal compound relative to the polyester, and M/P indicates a molar ratio of the total metal compound and phosphorus] and then effecting polycondensation.
As a process for alleviating the above problems of the direct polymerization process, Japanese Patent Application Laid-Open No. 62627/1984 proposes, in producing a polyester whose main repeating unit consists of ethylene terephthalate, according to the direct polymerization process, addition, before the start of esterification, of a Mg compound in an amount of 30-400 ppm (as Mg atom) relative to the polyester and addition, between the timing at which the esterification degree has reached 91% or more and the timing at which the initial condensation reaction has been completed, of a P compound and an alkali metal (Na, K) compound so as to satisfy the following formulas EQU 1.2.ltoreq.Mg/P.ltoreq.2.0 EQU 3.0.ltoreq.M.ltoreq.50
wherein Mg/P indicates an atomic ratio of Mg atom and P atom, and M indicates the amount of the alkali metal compound added, expressed in terms of metal atom (ppm) relative to the polyester.
With a polyester material obtained according to a conventionally known process, it is difficult to produce a high quality polyester film satisfying the market needs, at a high casting speed. When using a Ca compound, in particular, reduction in resistivity always gives rise to precipitation of large particles, making it difficult to completely prevent reduction in transparency.
As a process for alleviating the above problem, Japanese Patent Application Laid-Open No. 62626/1984 proposes addition, at the timing at which the esterification degree in the direct polymerization process has reached 91% or more, of a P compound, a Ca compound and an alkali metal compound selected from Na compounds and K compounds, in this order and so as to satisfy the following formulas EQU 50.ltoreq.Ca.ltoreq.400 EQU 1.2.ltoreq.Ca/P.ltoreq.3.0 EQU 3.0.ltoreq.M.ltoreq.20
wherein Ca indicates the amount of the Ca compound added, expressed in terms of Ca atom (ppm) relative to the polyester, Ca/P indicates an atomic ratio of Ca atom and P atom, and M indicates the amount of the alkali metal compound added, expressed in terms of metal atom (ppm) relative to the polyester.
To reduce the resistivity of a molten polyester, it is necessary to add thereto large amounts of an alkali metal compound and an alkaline earth metal compound. The addition, however, invites precipitation of particles in the polyester and the particles become large. These large particles cause appearance of pin holes in formed film, arching during film formation, and striking deterioration in color of polyester film (this gives a polyester film of very yellowish color), all of which are undesirable. Hence, Japanese Patent Publication No. 40538/1986 proposes addition to a polyester, of 0.01-2% by weight of a fine inorganic powder and 0.0001-0.0025% by weight (as metal) of an alkali metal compound soluble in ethylene glycol.
All of the above proposals are intended to increase the amount of electrostatic charges deposited on the surface of a film-like polyester melt by reducing the resistivity of a molten polyester. However, increasing electrostatic adhesion by reducing resistivity has a limitation.
Japanese Patent Application Laid-Open No. 187724/1987 and Japanese Patent Application Laid-Open No. 189133/1987 give the following description. Maximum casting speed increases largely with decrease in resistivity until the resistivity decreases down to about 0.2.times.10.sup.8 .OMEGA..multidot.cm; however, in the range of maximum casting speed of 50 m/min. or more and in the range of resistivity of 0.5.times.10.sup.8 .OMEGA..multidot.cm or less, the difference of resistivities at a maximum casting speed of 70 m/min, and at a maximum casting speed of 80 m/min. is only 0.01.times.10.sup.8 .OMEGA..multidot.cm and the correlation between maximum casting speed and resistivity becomes very poor. The above two documents (Japanese Patent Application Laid-Open No. 187724/1987 and Japanese Patent Application Laid-Open No. 189133/1987) propose use of a molten polyester having an initially accumulated electric charges amount of 2.9 .mu.m/mm.sup.2 or more and, as a means of increasing the initially accumulated electric charges, addition of a Mg compound and a P compound so as to satisfy the following formulas EQU 30.ltoreq.Mg.ltoreq.400 EQU 0.8.ltoreq.Mg/P.ltoreq.3
Mg indicates the content of the Mg compound, expressed in terms of Mg atom (ppm) relative to the polyester, and Mg/P indicates an atomic ratio of Mg atom and P atom), or addition of said compounds and at least one of Co, Na, K and Zr compounds in combination in particular proportions.
In these cases, also, there are used, as mentioned above, a Mg compound, a P compound, and Na, K, Co or Zr compound(s) in order to increase the initially accumulated electric charges amount; therefore, it is difficult to prevent increase in large particles, deterioration in polymer color, etc. caused by increase in amounts of said compounds.
Meanwhile, Japanese Patent Publication No. 22334/1972 proposes a process for produecing a polyester having improved dyeability to basic dyes, by adding to a polyester a phosphonium sulfonate represented by the following formula ##STR1## (A is an aliphatic group or an aromatic group; Y.sup.1 is a carboxylic acid ester-forming group; Y.sup.2 is the same group as Y.sub.1 or a hydrogen or halogen atom; R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are a group selected from the group consisting of hydrogen atom, alkyl group, aryl group and hydroxyalkyl group; and l is a natural number) in an amount of 10 equivalent % or less based on the repeating unit of the polyester.
U.S. Pat. No. 3,732,183 discloses a filament-forming aromatic polyester dyeable with cationic dyes, containing a sulfonate group represented by the following formula ##STR2## [R is an alkyl group, for example, CH.sub.3, C.sub.2 H.sub.5, n--C.sub.3 H.sub.7, n--C.sub.4 H.sub.9, 3-sodium(sulfopropyl) group and 4-sodium(sulfobutyl) group] in an amount of 0.5-5 mol % based on the constituent unit of the polyester.
Japanese Patent Publication No. 29500/1989 proposes, as an antistatic agent for polymeric material, a compound represented by the following formula ##STR3## wherein A is an alkyl or alkenyl group of 4-18 carbon atoms, a phenyl group, a C.sub.1-18 alkyl-substituted phenyl group, a naphthyl group or a C.sub.1-18 alkyl-substituted naphthyl group; and R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are the same or different and a hydrocarbon group of 1-18 carbon atoms or
a substituted hydrocarbon group of 1-18 carbon atoms. The above document discloses, as the polymeric material, a poly(ethylene terephthalate) and recommends 0.1-10% by weight as the amount of antistatic agent added.
The following laid-open publications which were laid-open to public later than the priority date of the present application, also discloses cationic dyeable polyester fibers using compounds similar to the phosphonium sulfonates disclosed in Japanese Patent Publication No. 22334/1972, U.S. Pat. No. 3,732,183 and Japanese Patent Publication No. 29500/1989.
European Patent Application Laid-Open No. 0280026 discloses an aromatic polyester fiber containing 0.1-10 mol % of a phosphonium sulfonate represented by the following formula ##STR4## wherein A is an aromatic or aliphatic group; X.sup.1 is a hydrogen atom or an ester-forming group; X.sup.2 is an ester-forming group which is the same as or different from the ester-forming group of X.sup.1 ; R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are independently an alkyl or aryl group; and n is an integer of 1 or more.
Japanese Patent Application Laid-Open No. 103650/1989 discloses a process for producing a cationic dye-dyeable polyester composition showing excellent heat resistance during melt spinning, by incorporating 0.002-2 mol % of an alkali metal or alkaline earth metal salt of an organic carboxylic acid into a polyester modified with 0.05-20 mol % of the same phosphonium sulfonate as the general formula mentioned in the above European Patent Application Laid-Open No. 0280026.
Japanese Patent Application Laid-Open No. 103623/1989 discloses a process for producing a cationic dyeable polyester showing excellent heat resistance during melt spinning, by incorporating, in place of the metal salt of an organic carboxylic acid used in the above Japanese Patent Application Laid-Open No. 103650/1989, 0.1-50 mol % of an aromatic dicarboxylic acid represented by the following formula EQU HOOC--B--COOH EQU wherein B is ##STR5##
Japanese Patent Application Laid-Open No. 192823/1989 discloses a polyester composition containing 0.01-20 mol % of the same phosphonium sulfonate as the general formula disclosed in the above Japanese Patent Application Laid-Open No. 103650/1989, 0.05-10% by weight of a metal sulfonate represented by the following general formula EQU (RSO.sub.3).sub.n M
(R is an alkyl group of 6 or more carbon atoms, an aryl group or an aralkyl group; M is an alkali metal or an alkaline earth metal; and n is 1 or 2) and 0.05-10% by weight of a poly(oxyalkylene glycol) and/or its derivative. According to the description of the document, this polyester composition prevents generation of static electricity during melt molding and has excellent moldability.
Lastly, Japanese Patent Application Laid-Open No. 197523/1989 proposes a process for producing a modified polyester of excellent moldability and color, containing 0.005-0.495 mol % of the same phosphonium sulfonate as the general formula disclosed in the above Japanese Patent Application Laid-Open No. 103650/1989 and 0.005-0.495 mol % of a metal sulfonate represented by the following formula ##STR6## (Z.sub.1 is an aromatic or aliphatic group; A.sub.1 is an ester-forming functional group; A.sub.2 is a hydrogen atom or an ester-forming functional group which is the same as or different from the ester-forming functional group of A.sub.1 ; M is a metal atom; and m is a positive integer) so that the total of the two sulfonates become 0.01-0.5 mol %.
In recent years, in magnetic recording media which are the main usage of biaxially stretched polyester film, there are required very high recording density and higher quality video image. Accordingly, said film is required to have excellent smoothness. Large particles present in film which cause, for example, drop-out, reduce film quality significantly. Further, according to the research by the present inventor, a film of a polymer containing large amounts of an alkali metal compound and an alkaline earth metal compound comes to possess therein a large number of black foreign matters, and these black foreign matters significantly reduce the quality of the film.